The present invention relates to a power semiconductor device, in particular to vertical power devices with guard rings.
Power semiconductors are mostly used to switch high currents and high voltages in many applications. They have to be stable for a long time. Depending on the application, the devices have to switch frequently or only occasionally. The requirement for power devices generally is to have very low forward voltage drops in conduction mode and a very low leakage current in blocking mode.
Usually power semiconductor devices include n-type and p-type conductivity regions, metalized contact areas for current conduction and passivated areas configured to handle a given breakdown voltage.
Some power devices are configured to handle up to several thousand volts in blocking mode. As a result, the pn junctions in such devices produce very high electric fields which penetrate the surface of the devices. To withstand these blocking voltages, the power devices may be provided with special guard ring structures to reduce the electric fields stepwise.
Generally, the actual breakdown voltage is influenced by the design of the guard rings as well as the charges at the upper surface of the semiconductor substrate. The ions and charged particles in the insulating layers overlying the depletion region also negatively affect the breakdown voltage.
To obtain stable devices, the charges on the depleted zones should be time and temperature independent. Non-passivated depletion areas generally cannot be handled. Typically, the devices are mounted in plastic housings with potting materials (which includes glue or adhesion material). Such materials can be highly ion contaminated and/or may be polarized by the electric fields of the power devices. To protect the devices against these influences from the environment, they are covered with passivation layers on top of the depletion zones.
Also, ion contaminated glues should be kept away from the silicon surface, preferably as far as reasonably possible. Any coating or layer that is in direct contact with the silicon surface should be of very high purity in order not to degrade the breakdown voltage rating as well as for other reasons.